Available online at www.sciencedirect.com
ScienceDirect Procedia Technology 26 (2016) 136 – 143
3rd International Conference on System-integrated Intelligence: New Challenges for Product and Production Engineering, SysInt 2016
Development of a combined measurement system for torque and angular position T. Menkea,*, C. Ungerb, A. Daia, D. Kramera, B. Eilerta, G. Ullmanna, L. Overmeyera a
Institut fuer Integrierte Produktion Hannover gGmbH, Hollerithallee 6, 30419 Hanover,Germany b Laser Zentrum Hannover e.V., Hollerithallee 8, 30419 Hanover, Germany
Abstract In this article a combined contactless measurement method is presented which is based on angle differences. The aim is the development of a combined, optical measurement system to determine the angular position of a shaft and the applied torque on it as well as an appropriate production technology to apply markings. Two independent modules are used which separately allow the measurement of angular position and rotational speed and if combined can measure torque. To ensure a simple integration of the system into any application, position markings are directly applied on the shaft using a laser. The selected technological approach is based on a contactless measurement method using angle differences. The concept as well as first research results are presented. ©©2016 Published by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license 2016The TheAuthors. Authors. Published by Elsevier (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SysInt 2016. Peer-review under responsibility of the organizing committee of SysInt 2016 Keywords: absolute angular position; angle difference; contactless combined measurement; sensor; torque
1. Introduction For precisely controlling automated systems as well as monitoring their power, knowledge of current torque and rotational speed is an essential prerequisite. Furthermore storing these two measured values over the system’s
* Corresponding author. Tel.: +49-511-27976-230; fax: +49-511-27976-888. E-mail address:
[email protected]
2212-0173 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of SysInt 2016 doi:10.1016/j.protcy.2016.08.019
T. Menke et al. / Procedia Technology 26 (2016) 136 – 143
lifetime offers the possibility of long-term condition monitoring. This paper presents the concept of a novel noncontact measuring method to capture the absolute angular position twice and thus torque. Existing industrial applicable measuring systems that detect rotational speed and torque do not offer combined, non-contact and direct measuring methods. Non-contact implies that it is not necessary to apply strain gauges or incremental disks and that measured values are transmitted without any contact with the specimen. Most measuring methods detect absolute angular position and torque separately by means of rotary encoders and torque transducers. This separate detection, however, results in some disadvantages: often the measuring devices are incompatible to each other, inaccurate, require a lot of installation space and increase the weight and costs of the entire system. Furthermore, in most cases existing measuring systems require constructive changes and additional attachments to the measuring shaft which increases the weight as well as the installation effort. Nomenclature γ e G IT l m M n φ R
shearing angle number of markings/Increments shear modulus torsional moment of inertia length (of torsion) length of the code word torque rotational speed angle of twist radius
1.1. Requirements to the system The design requirements of the system shown in Table 1 were identified in collaboration with potential users of well-known companies. The measuring system to be developed should cover a wide measuring range with high resolution and accuracy at both low and high rotational speeds. For extensive practical use, the requirements were divided into minimum requirements and ideal requirements. Table 1. Requirements to the measuring system. Description
Minimum requirements
Ideal requirements
Type of shaft
solid shaft
solid and hollow shaft
Surrounding
low vibration and dust, lubricant-free
fuel/oil
Measurement method
non-contact
non-contact
Measuring range of torque [Nm]
- 50…+ 50
- 50…+ 50
Shaft diameter [mm]
10 - 20
> 10
Length of torsion [mm]
> 50
> 50
Range of rotational speed [rpm]
0 - 1000
0 - 12000
Resolution measurement of rotational angle [°]
< 0.01
< 0.01
Accuracy measurement of rotational angle [%]
< 0.1 FS
< 0.1 FS
Interface
digital
digital
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1.2. Relation between torsion and twist For the purpose of illustration, a torsion bar of the length l and radius R is shown in Fig. 1. Applied torque Mz leads to a torsion of the bar. The resulting twist of the shaft can be characterized by the shearing angle γ or the angle of twist φ.
Figure 1. Torsion of a solid cylinder.
At steady torque MT and constant torsional stiffness GIT, the angle of twist (in degrees) of a bar is obtained from the following equation [1, 2]:
M
M T l 180 GIT S
(1)
The polar moment of inertia or torsional moment of inertia IT of solid cylinders is calculated as follows [1, 2]:
IT
S 4 R 2
(2)
At an applied torque of MT = 0 Nm the shaft is not twisted (φ = 0°). Using a shaft made of ductile steel 42CrMo4 (shear modulus G = 81,000 N/mm2) with a diameter of d = 20 mm and a length of l = 100 mm, a torque of MT = 100 Nm results in a twist angle of φ = 0.45°. 2. State-of-the-art Various methods to measure torque, rotation speed or rotation angle separately are found in literature, and therefore are not described in detail at this point. But it needs to be mentioned that a trivial interconnection of existing technologies is not possible for technological and economical reasons. Hence the development of a combined system for measuring torque and rotational speed is desired. In the following, two combined measuring methods are introduced. In [3] a measuring system consisting of two sensor ball bearings is described that detects the temporal deferral and thus the applied torque via the position of modified rotating inner rings. At a speed of n = 800 rpm and a maximum torque of M = 10 Nm a deviation of less than 1.5% is achieved. In [4] a combined quasi-non-contact method is reported to detect torque and rotational speed. The power can be derived from these measurements. One trigger transducer and two speed transducer disks are mounted on the shaft, captured by one trigger sensor and two speed sensors. After calibration, the comparison of the phase position indicates the applied torque and the evaluation of the speed signals enables the identification of rotational speed and thus the power. Measured values are captured without contact, but require additional attachments to the shaft due to
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the transducers. At a speed of n = 1670 rpm and an applied torque of M = 1008 Nm an accuracy of up to 0.06% is achieved. Both mentioned methods are not able to measure torque and rotational speed at a standstill and to detect the absolute angle. Furthermore no (completely) contactless and combined technology is available to measure torque and the angle of rotation at the same time. 3. Setup of the system The solution presented in this paper (Fig. 2) is based on measuring absolute rotational angle twice and thus torque by two independent measuring modules (imaging module) in combination with a processing module. In each case the imaging modules capture the markings applied to the shaft (solid cylinder). The markings are applied using a laser process and serve as location encodings, this is explained in more detail in chapter 4. By the use of the two measuring modules at a distance l from each other the absolute rotation angle φ1(z1) and φ2(z1 + l) are detected. Using equation 1 the applied torque can be calculated from the angular difference. Furthermore the rotational speed and thereby the power can be determined. The measuring modules in each case consist of a CMOS-sensor, a pulsed LED, a beam splitter and corresponding optics for imaging, focusing and image correction. The LED’s light is guided to the shaft, reflected and then detected by a CMOS-Sensor. Depending on the shaft’s rotational position a different area of the coded markings is visible, which can be imaged directly and processed by the processing module. A short exposure time by means of short LED pulses allows sharp images at high rotational speeds of the shaft, as they e.g. appear in gearboxes or turbines.
Figure 2. Setup of the system. (simplified representation)
4. Concept of the angle encoder system Current non-contact rotary encoders are mainly based on inductive, magnetic or optical technologies that can be divided in incremental or absolute angle transmitters. In each case these show different advantages and disadvantages. For example, inductive rotary encoders are relatively compact, magnetic encoders are resistant to moisture and optical systems show high accuracy and high compatibility to magnetic fields [5]. Optical measurement methods of detecting rotation angles are often combined with so-called code disks that are permanently fixed to the shaft and therefore rotate with the same period. Code disks of incremental angle transmitters have alternating transparent (corresponding to binary 1) and non-transparent or non-reflecting (corresponding to binary 0) markings e of uniform width, that are applied in circumferential direction in a single track [5]. The angular resolution follows from 360/e [6]. Absolute angle transmitters detect the angle with the help of local codings. This is realized by either one single track (so-called single track coding) or multiple tracks (socalled multiple track coding). For a single track coding a non-trivial coding is used so that several markings together
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make up unique angles. Examples for multiple track codings are binary coding or the so-called Gray-Code. More information about track coding and rotary encoders are given in [7,8]. 4.1. Requirements to the codings to be developed The markings used for local coding need to fulfil certain requirements, which are described in the following. Because the markings are applied with the help of a laser, a binary coding by blackening (corresponding to binary 0) and non-blackening (corresponding to binary 1) of the surface is considered initially. To recognize the absolute angle it is required that every sequence of m markings (m = log2 e) [6] can be clearly identified to ensure no ambiguity. Furthermore a clear transition from the ending to the beginning of the markings (closed loop) has to be ensured. To minimize the effort for marking and related possible sources of errors or inaccuracies, a single track coding was selected and tested. Because many different applications and shaft diameters are possible it is necessary to select the number of markings e flexibly. Alternatively, the line width could be varied in laser processing, but this is initially disregarded since processing parameters are optimized for one line width (one line is corresponding to a binary zero-marking). An unrolled sample code to meet these requirements is shown in Fig. 3 with e = 16 and hence the considered code word length is m = 4. Any four (or more) connected markings can be allocated clearly within the whole coding. For illustrating purposes of the sample, the explicit code word 0110 is highlighted, which complies with position 6. On the basis of the position, the rotation angle can be identified directly. The three markings (000) at the end show the seamless transition of the ending and beginning of the markings.
Figure 3. Unrolled random-code and tabular view.
4.2. Programming the coding The procedure of programming is shown in Fig. 4 which starts with the user’s input of the number of increments e and hence the exponent m can be calculated. Then an array of equal length is created and expanded step by step. This is followed by loops of changing the array and testing if the required length is reached or if the code is unambiguous. Finally a machine code for the laser marking system is generated. The laser system applies the code to the shaft fully automated.
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Figure 4. Flow chart of the program.
5. Results and Discussion In order to validate the developed coding, a simulation software was programmed that displays the unrolled code step by step with the help of a monitor. For the purpose of simulation a coding with e = 2000 was chosen. A webcam positioned towards the monitor captures the entire surface of the monitor and transmits the image to a PC. Then image processing is done which is similar to the subsequent implementation. In the course of image processing, a decoding of the imaged code is realized. With the image capture and processing being completed, the imaged code segment is shifted by 1° and the procedure is repeated. The simulation determines an associated angle φdetermined for every taken image and compares that angle to the previously given angle φpreset. Each angular deviation (Δφ = | φpreset - φdetermined|) is shown in a normalized form in Fig. 5 for the fully unrolled code (0-360°). The absolute angular deviation is not shown here since it is of no relevance due to the current experimental setup. However, it has to be mentioned that the imaged angle deviation is much smaller than one (Δφ
